For example, in a system of Patent Document 1, a particulate matter sensor (a PM sensor) for detecting particulate matters (hereinafter referred to also as “PM”) in an exhaust passage of an internal combustion engine is arranged. This PM sensor includes an insulating substrate, and a pair of electrodes that are arranged, spaced apart from each other, on the insulating substrate.
If the PM in exhaust gas is deposited between the pair of the electrodes of this PM sensor, the conductivity between the electrodes changes. There is a certain correlation between the amount of the deposited PM and the conductivity between the electrodes, and the resistance between the electrodes changes in accordance with the amount of the PM deposited between the electrodes. Besides, there is a correlation between the amount of the PM deposited between the electrodes and the amount of the PM in exhaust gas. Accordingly, the amount of the PM in exhaust gas is detected by detecting a resistance value between the electrodes of the PM sensor.
By the way, in the art of Patent Document 1, the PM sensor is arranged downstream of a particulate matter collection filter (a diesel particulate filter; hereinafter referred to also as “a DPF”). In Patent Document 1, it is determined whether or not there is a malfunction in the DPF, by detecting an amount of the PM discharged downstream of the DPF on the basis of the resistance value between the electrodes of the PM sensor.